ValidateModelStateAttribute Action Filter

ValidateModelStateAttribute.cs

public class ValidateModelStateAttribute : ActionFilterAttribute {

    public override void OnActionExecuting(HttpActionContext actionContext) {

        var modelState = actionContext.ModelState;

        if (!modelState.IsValid) {

            var errors = modelState.Keys
                .Where(key => modelState[key].Errors.Any())
                .Select(key => new Dictionary<string, string> { 
                    { key, modelState[key].Errors.First().ErrorMessage }
                });

            //this way, conneg is done by the server
            var response = actionContext.Request.CreateResponse<IEnumerable<Dictionary<string, string>>>(
                HttpStatusCode.BadRequest, errors
            );

            actionContext.Response = response;
        }
    }
}